In the medical field, in view of environmental protection and space saving, reduction in waste processing solution has been largely expected in recent years. As to photothermographic materials for medical diagnosis and photographic techniques, this demands the progress of techniques which makes it possible to efficiently expose the photographic materials to light by use of a laser imagesetter or a laser imager and to form black images having high resolving power and sharpness. In these photothermographic materials, a heat developable processing system which reeds no processing chemicals for a solution system and can be more easily processed without impairing environment can be supplied for customers.
On the other hand, the techniques for semiconductor lasers which are recently making rapid progress have enabled medical image output devices to be miniaturized. Consequently, the techniques for infrared ray-sensitive photothermographic materials for which semiconductor lasers can be used as light sources have also been developed. The techniques for spectral sensitization for the materials are disclosed in JP-B-3-10391 (The term "JP-B" as used herein means an "examined Japanese patent publication"), JP-B-6-52387, JP-A-5-341432 (The term "JP-A" as used herein means an "unexamined published Japanese patent application"), JP-A-6-194781, and JP-A-6-301141, and further, the techniques for antihalation are disclosed in JP-A-7-13295 and U.S. Pat. No. 5,380,635. In the photosensitive materials for which exposure to infrared rays is a prerequisite, sensitizing dyes and antihalation dyes are allowed to have largely low absorption in the region of visible light to easily prepare substantially colorless photosensitive materials.
However, spectral sensitizing dyes absorbing infrared rays have a tendency to reduce silver ion in the photosensitive materials and suffer deterioration in fogging, because these dyes generally possess strong reducing power due to high HOMOs thereof. In particular, these photosensitive materials have the disadvantage of undergoing marked changes in performance on storage under the conditions of high temperature and high humidity or on storage for a long period of time. Use of dyes possessing lower HOMOs, in which the LUMOs also become relatively low, results in decreasing spectral sensitization efficiency to cause sensitivity decrease of the photosensitive materials. Such disadvantages in sensitivity and storability are more significant in photothermographic materials to which the present invention relates than in wet type photographic materials.
It is a matter of course that use of large quantities of the dyes increases reducing capacity of the dyes. However, use of small quantities of the dyes results in poor sensitivity because of insufficient absorption of light incident on the photosensitive materials. In particular, in the photothermographic materials in which binders having strong affinities to oil are used, weak adsorption of the dyes to silver halides which are photosensitive elements causes poor sensitivity, unless sufficient quantities of the dyes are added.